Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In a typical market area, wireless service providers may operate radio access networks (RANs) each arranged to provide user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, with wireless communication service. Each such a RAN may include a number of base stations that radiate to define wireless coverage areas in which to serve UEs according to an air interface protocol such as Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) or Wireless Operability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1xRTT and 1xEV-DO), GSM, GPRS, UMTS, EDGE, iDEN, TDMA, AMPS, MMDS, WIFI, and BLUETOOTH, or others now known or later developed. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the RAN may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station or by other base stations.
In general, each base station in a RAN may be configured to operate on one or more frequency bands and, in each such band, to provide service on one or more frequency channels. Example frequency bands, each defining a particular range of radio frequency spectrum, include the 800 MHz band, the 1.9 GHz band, and the 2.5 GHz band. Each frequency channel in a band may then be arranged as a specific block of frequency, such as a 1.25 MHz block, a 5 MHz block, a 10 MHz block, or the like. Further, in certain systems, each frequency channel may include one block of frequency for downlink communications from the RAN to UEs and a corresponding but separate block of frequency for uplink communications from UEs to the RAN. Whereas, in other systems, each frequency channel may be divided over time to separately carry downlink and uplink communications.
To enable UEs to select coverage areas in which to operate, each base station may also be configured to broadcast a specification of the one or more frequency channels on which it operates and to broadcast on each such frequency channel one or more pilot or reference signals that UEs are configured to monitor in order to evaluate coverage strength (e.g., signal strength and/or signal to noise ratio). For example, in a representative LTE system, each base station broadcasts on each of its one or more frequency channels a system information block (SIB) message that lists the frequency channels on which the base station provides service, and each base station further broadcasts on each listed frequency channel a reference signal that LTE-compliant UEs are arranged to monitor in order to evaluate LTE coverage provided by the base station. Similarly, in a representative CDMA system, each base station broadcasts on each of its one or more frequency channels a channel list message (CLM) that lists the frequency channels on which the base station provides service, and each base station further broadcasts on each listed frequency channel a pilot signal that CDMA-compliant UEs are arranged to monitor in order to evaluate CDMA coverage provided by the base station.
When a UE first enters into coverage of a RAN, the UE may automatically scan the airwaves in an effort to find the strongest available coverage, and the UE may then register with the RAN. For instance, the UE may generally scan through various frequency channels and evaluate any reference signals on those frequency channels, and the UE may determine which reference signal is the strongest, thereby identifying an optimal frequency channel and base station coverage area. The UE may then engage in registration signaling with the RAN on that frequency channel in that base station coverage area, to register with the RAN.
Once a UE is registered with a RAN in a particular coverage area, the UE may then be served by the RAN in that coverage area in an “idle” mode in which the UE regularly scans for page messages and other overhead information from the RAN. Further, the UE may engage in additional signaling with the RAN in order to be served by the RAN in an “active” or “connected” mode in which the UE may then engage in bearer communication, such as voice and/or packet-data communication with other entities as discussed above.
While a UE is served by a RAN in the idle or active/connected mode, the UE may also continue to scan the airwaves to help ensure that the UE continues to operate with the most optimal coverage. For instance, the UE may continue to evaluate the reference signal of the UE's currently serving frequency channel and coverage area, and if that reference signal becomes threshold weak, the UE may begin to scan for coverage on one or more other frequency channels of that coverage area and/or on one or more frequency channels of one or more other coverage areas that the UE can detect. Alternatively, the UE may regularly engage in such scanning for other coverage without first detecting threshold weak coverage on its currently serving frequency channel. In the event the UE finds sufficiently strong other coverage, such as another frequency channel and/or other coverage area, the UE may then transition to be served by the RAN in that other coverage.
Many market areas today also include two or more such RANs arranged to operate according to different air interface protocols. In such areas, UEs may then be configured as multi-mode devices, arranged to support service according to various air interface protocols and thus arranged to be served by more than one RAN in the area. By way of example, a given market area may include both an LTE RAN operated by one wireless service provider and a CDMA RAN operated by the same or another wireless service provider, and UEs in the area may be configured to support both LTE service and CDMA service. Such a UE may be configured to support service by just one such RAN at a time or may be configured to support service concurrently by both RANs.
With such an arrangement, while a UE is being served by one RAN in the idle or active/connected mode, the UE may begin being served by another RAN. This may occur, for example, if the two RANs support different types of services and the UE is served by just one RAN currently but, to engage in another service, needs to begin being served by the other RAN, or for other reasons. In the LTE/CDMA arrangement, for instance, LTE generally supports data communication including in some cases packet-based voice calling, and CDMA supports legacy circuit-switched voice calling. Thus, if a UE is currently being served by an LTE RAN for data service and seeks to engage in a legacy voice call, the UE may scan for and begin being served by a CDMA network in order to engage in that voice call.
Without limitation, a specific example of this process is known in the industry as circuit-switched fallback (CSFB). With CSFB, a UE may be registered with and served in an idle or active/connected mode by a first RAN, and the UE may also be registered with a co-located second RAN but not currently served by that second RAN. While the UE is served by the first RAN, signaling between the UE and the second RAN may then pass through the first RAN, to cause the UE to begin being served by the second RAN. For instance, in an example LTE/CDMA arrangement, when the UE is registered with and served by the LTE RAN, the UE may engage in CSFB signaling with the LTE RAN via the LTE air interface, and the LTE network may pass that signaling along to the CDMA network. When appropriate, the UE may then scan for optimal CDMA coverage and may enter into an active/connected mode served by the CDMA RAN, to engage in the CSFB call via the CDMA air interface.